I'd say the fastest decline in a MMORPG I've seen was with Final Fantasy XIV. It looks like that game won't be close to back on track until 2013.

I have high hopes for SWTOR still. 1.3 looks promising and the content they showed off at E3 looks fun too. Hopefully the game doesn't fail miserably. If BioWare learns from their many mistakes this game can still have a long prosperous life.